There are generally two reasons why people take drugs. Either they are addicted, in which case they are seeking to avert the negative consequences of withdrawal, or they are not addicted, in which case they are seeking the positive effects of intoxication. As far as we know, none of the senior politicians who have confessed to, or been accused of, drug use falls into the first category.

On the Labour benches, Alistair Darling, Jacqui Smith, Andy Burnham, Ruth Kelly, Hazel Blears, Harriet Harman, John Hutton, Patricia Hewitt, Yvette Cooper, John Denham, Charles Clarke and Tony McNulty have all admitted smoking cannabis at some point in their lives. Six of them are serving members of the cabinet. On the Conservative benches, there have been allegations, unproven, that both David Cameron and George Osborne have taken cocaine. On the subject, David Cameron said: 'All I have said about my past... is that what is private in the past should remain private.' Tim Yeo and David Willetts have said they tried cannabis. The Tories' most powerful elected official, Boris Johnson, has admitted trying cocaine and smoking 'quite a few spliffs' at university.

But there is no evidence that any of them has had their will crippled and their judgment warped by the desperate need to satisfy an illegal appetite. Perhaps that is why none of the politicians who has confessed has suffered as a result. Since 27 per cent of British adults have taken some form of illegal drug it would make sense for the public to be relaxed about aspiring leaders flirting with narcotics. Youthful experimentation is not a sacking offence.

The only politician in recent years whose career has been damaged by substance abuse is Charles Kennedy. He was ousted as leader of the Liberal Democrats in January 2006 because of an alcohol addiction. Booze is not illegal. Kennedy was punished for his habit because it was impairing his performance and forcing his colleagues into serial dishonesty. He appeared at an election campaign press conference in 2005 dishevelled and incoherent, for example. Covering up the problem was a liability for the party.

If any politician was exposed as a chronic junkie, his or her career would be over immediately. The fact of addiction would be a disqualification from competent government. But that is a matter of capability, not morality. So is the fact that drugs are illegal the only reason politicians can't take them, or rather, be seen to be taking them?

Since the Sixties, recreational drug use has become progressively more common, so we can assume that successive cohorts of politicians will include more occasional drug takers. It might reasonably follow that drug laws would become progressively more liberal. Cannabis was indeed downgraded from a class B to class C drug in 2004 - the first significant relaxation since the classification system was first introduced in 1971.

But cannabis is an exceptional case. The change in law followed a long period in which pot smoking had become normalised, with enthusiastic depictions of its effects in film, on TV and in pop music. The marijuana leaf had become a fashion symbol. Politicians only broke cover once it was clear that there was little danger of public outrage if they admitted having used the drug. Ministers did not have to defend reclassification with reference to their empirical experience; they were dealing with a cultural fait accompli. Sure enough, as evidence emerges that modern 'skunk' strains of cannabis are much stronger than the old weed feted in popular culture, re-reclassifaction to Class B is on the agenda.

But what about really 'hard' drugs? Of those Britons who have dabbled at all, more than a third have taken ecstasy and cocaine. If a substantial minority of voters have flirted with class A substances and come away unscathed, surely they would forgive similar risk-taking among their putative leaders. That certainly seems to be the case across the Atlantic. In his first volume of political memoirs, Barack Obama admits that he and his friends did 'a little blow [cocaine], when we could afford it'. And Americans are generally assumed to moralise more in their judgment of politicians than Britons.

If David Cameron fails to become our next Prime Minister it is unlikely to be unsubstantiated rumours of drug use in his youth that are to blame. If anything, the whiff of subversion in his past served an accidental but expedient function when he first became Tory leader. The Conservative party at that time was routinely caricatured as dislocated from the cultural mainstream and entrenched in bitter reaction against modern Britain. Cameron stated that his primary task was to 'detoxify the brand'. Officially, that meant making speeches about poverty and being photographed on eco-missions to the Arctic. But a cocaine non-scandal probably did no harm. The suggestion of contact with drugs culture at least opened up the possibility that the Tory leadership, in its youth, had lived on the same planet as the rest of us. That was a substantial advance for the party in 2005.

Of course, the idea that politicians should, by their social and cultural experiences, resemble the people they govern has always been a fiction. Being 'out of touch' is an absurd charge to level against people who, by definition, occupy a rarefied stratum of an atomised society. With which particular demographic segment are they supposed to be 'in touch'?

It is a particularly ridiculous aspiration today, when anyone who enjoys a high public profile can expect details of their private life to be trawled for evidence of misjudgment, hypocrisy or ethical turpitude.

For dilettantes who go into politics after a career in some other field that leaves hostages to fortune. But for a generation of professional, ambitious young hacks who manoeuvre their way from campus soap boxes, via think-tanks and a stint in party research departments, to winnable seats and positions on the front benches, safety has to come first. Presumably, as the spliffs are going round in undergraduate digs these days, there are people who hesitate to partake just in case they end up pictured on Facebook somewhere, with a future career-wrecking reefer in their hand.

We only pretend to want politicians to be ordinary people. In reality, we still think that the responsibilities they hold make it desirable (if optimistic) that in matters of self-discipline and sobriety, they are extraordinary.

So it is dangerous for politicians to admit to drug use, even when it has become less taboo in mainstream culture. They are not celebrities, even when subject to the same kind of media scrutiny. It is just about OK for Kate Moss to be pictured snorting lines of cocaine in a recording studio, because she won't have to formulate fiscal policy the next morning.

Ostensibly the law proscribes certain substances for mundane practical reasons, because their consumption harms individuals and communities. But there is still something essentially puritan in our national attitude to drugs: intense gratification should follow the application of effort. It should be a reward for virtuous behaviour. Getting it from intoxication, legal or otherwise, is a kind of moral laziness. We accept that kind of petty corruption in ourselves, but we like to think that politicians are different; that they are acting in a higher interest and so are more capable of the kind of discipline required for abstinence.

So even when politicians have confessed to drug use, they rarely, if ever, admit to having enjoyed it. Most enter caveats laden with regret. 'It was wrong that I smoked it when I did,' said Jacqui Smith. 'She recognises that it was a foolish and silly thing to do,' a spokesman said on behalf of Ruth Kelly. 'I didn't like it,' said David Willetts. 'I don't want my kids to take drugs,' Boris Johnson stipulated.

Those responses define drug use as a mistake that the public should forgive, as long as suitable contrition is expressed. That approach is conditioned in large part, of course, by the technical awkwardness of members of the legislature disobeying the rules. Hence, David Cameron's carefully worded denial that he had taken drugs since becoming an MP: 'Lawmakers should not be lawbreakers.'

That is a neat formula, but as a moral position it is meaningless. Most drug users don't sit in Parliament. Presumably, MPs don't think that ordinary punters' lack of legislative obligation should leave them freer to break drug laws than public servants. If, say, Jacqui Smith had decided to pursue a career in circus juggling, would she be less judgmental about her old self? If significant numbers of politicians have disregarded a law in the past, you might reasonably expect them to debate the functional necessity and practicality of that law.

But instead they express sinner's penitence and defer to the moral authority of the law as it stands. In so doing they dodge the vital question about their personal experience. Why do they think they escaped unscathed from contact with drugs, when others did not?

They know the answer: the fact of them having ambitions to high public office kept any miscreant tendencies in check. Most of the leading politicians in this country who have taken drugs did so at university, many at Oxford and Cambridge. That is, generally speaking, a pretty tame exposure to drugs culture. Like many middle-class users, the future Cabinet ministers were operating with an invisible cultural and financial safety net. They had too much invested already in the future to risk throwing it all away in drug-fuelled abandon. It was safer for them to break the law precisely because they were on a trajectory to be lawmakers.

That's the hidden reason why politicians never discuss drugs candidly. Of course, any candidate with narcotics in his past will have to undergo some symbolic purgation by media, which most would rather avoid. But that can be managed with a dose of contrite spin.

The real reason politicians hate talking about drugs isn't because they fear being perceived as reprobates and criminals. Quite the contrary, if they were honest they would admit how removed their experiences were from the real criminality of the drugs trade. They would have to acknowledge that it isn't necessarily the substance that harms, but the context in which it is taken.

But politicians' acknowledgment of why they came away unscathed from contact with drugs would be a rebuttal of the whole case for prohibition. It would suggest that, given the right cultural and economic incentives to moderation, some drug use is safe. The really toxic admission for a politician isn't having taken drugs, but having done so after clear-sighted evaluation of the risks; having enjoyed them, and having no regrets.

Politicians and their 'confessions'

Boris Johnson - no effect'I tried [cocaine] at university and I remember it vividly. It achieved no pharmacological, psychotropic or any other effect on me whatsoever.'

Oliver Letwin - I blame my friends'At Cambridge, I was a very pretentious student. I grew a beard and took up a pipe. Some friends put some dope in a pipe I wassmoking. It had no effect.'

Yvette Cooper - I was a kid'I did try cannabis while at university, like a lot of students at that time, and it is something that I have left behind.'

Jacqui Smith - so long ago'I have [smoked cannabis] when I was at university. I think it was wrong that I smoked it when I did. I have not done for 25 years.'

Tim Yeo - no big deal'I was offered [cannabis] on occasion and enjoyed it. I think it can be a much pleasanter experience than having too much to drink. I found it agreeable.'

Tony McNulty - I 'encountered' it'I encountered it [at university], I smoked once or twice. I don't think many people who were at university at the time didn't at least encounter it.'